Self-healable polymer complex with a giant ionic thermoelectric effect

In this study, we develop a stretchable/self-healable polymer, PEDOT:PAAMPSA:PA, with remarkably high ionic thermoelectric (iTE) properties: an ionic figure-of-merit of 12.3 at 70% relative humidity (RH). The iTE properties of PEDOT:PAAMPSA:PA are optimized by controlling the ion carrier concentration, ion diffusion coefficient, and Eastman entropy, and high stretchability and self-healing ability are achieved based on the dynamic interactions between the components. Moreover, the iTE properties are retained under repeated mechanical stress (30 cycles of self-healing and 50 cycles of stretching). An ionic thermoelectric capacitor (ITEC) device using PEDOT:PAAMPSA:PA achieves a maximum power output and energy density of 4.59 μW‧m−2 and 1.95 mJ‧m−2, respectively, at a load resistance of 10 KΩ, and a 9-pair ITEC module produces a voltage output of 0.37 V‧K−1 with a maximum power output of 0.21 μW‧m−2 and energy density of 0.35 mJ‧m−2 at 80% RH, demonstrating the potential for a self-powering source.


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was obtained from the linear relationship between the thermovoltage and temperature difference from the four experiments with different temperature gradients (ranging from 0.5 ℃ to 2 °C). All samples were placed in an isolated chamber and evaluated at each RH level (from 70 to 90% RH). For repeated stretching cycle experiments, sample films were prepared on a 3M-VHB-4910 substrate. Identical instruments were used to measure and record the operation of the ITEC device and module with a 10 k load resistor.
Measurement of the ionic and electronic conductivity: An electrochemical impedance spectrometer (EIS; COMPACTSTAT, IVIUM echnologies) was used to evaluate the ionic and electronic conductivity. The amplitude of the AC voltage was 0.1 V, and the frequency ranged from 1 Hz to 250 kHz. The ionic resistance (Ri) and electronic resistance (Re) are obtained by fitting the Nyquist plot ( Supplementary Fig. 4) with the equivalent circuit model ( Supplementary Fig. 6) using the EIS analyzer software. The electrical conductivity was calculated using the following equation: corresponds to the ionic and electronic resistance, is the distance between the two gold electrodes, and is the cross-sectional area of the sample film. 1 The thickness of the samples was determined using a surface profiler (KLA Tencor, P6).
Determination of the dielectric constant and loss: The Nyquist plot of each film was recorded using EIS in the same manner as described above. The dielectric constant ( ′) and loss ( ") were calculated using the following equations: 2 and S4 where is the angular frequency (rad s -1 ), = / , is the real impedance, " is the imaginary impedance, and is the free-space permittivity (8.854187 × 10 −12 F m −1 ).
Determination of the net ion carrier concentration and net ion diffusion coefficient: The method for determining the net ion carrier concentration ( ) and ion diffusion coefficient ( ) was adopted from Bandara's method, 3 which uses a curve-based fitting of the dielectric constant-frequency curve using the following equation: where is the high-frequency permittivity, is the time constant (s), is the coefficient , and is a dimensionless unit corresponding to the ratio of half the distance between the two electrodes ( in cm) to the Debye length ( in nm). The fitting of was performed in the middle-linear regime to exclude the experimental error from the left and right ends. 4 The fitting was conducted by plotting ′ as a function of the frequency (on a logarithmic scale). A nonlinear fitting analysis was conducted using Origin© software. and were calculated using the following equations: where is the ionic conductivity (S m −1 ), is the Boltzmann constant (8.617333262145 × 10 −5 eV K −1 ), is the measurement temperature at RT (298 K), | | is the elementary charge of the particle (1.60217662 × 10 −19 C), and is the half-distance (~ 0.2 cm) between the two electrodes.
Characterization of SEM and EDS mapping: Scanning electron microscopy (SEM) and where is the specific heat capacity, is the density of the sample, and is the thermal diffusivity. The thermal diffusivity was measured by laser flash analysis (LFA, NETZSCH) at 30% RH and 25 °C. The thermal diffusivity is described by = 0.1388 / / , where is the thickness of the sample, and / is the time required to reach the half-maximum temperature. The specific heat capacity was measured by differential scanning calorimetry (DSC8000, Perkin Elmer). It was obtained using a comparative method based on the following equation: where is the maximum temperature, is the density, is the thickness, and the S6 superscripts ref and test correspond to the reference and test samples, respectively. The was calculated to be 1.897 J g -1 K -1 , was determined to be 0.118 mm 2 s -1 , and was determined to be 1.454 g cm -1 , which produced of 0.325 W m -1 K -1 . The thermal conductivity of the sample was affected by the RH. The values of the samples at various RH values were obtained using the following equation: where ø is the volume fraction, and the subscripts , and correspond to the hydrated PEDOT:PAAMPSA:PA, PEDOT:PAAMPSA:PA at RH 30%, and water, respectively. The value of water was 0.6 W m -1 K -1 . Supplementary Fig. 11 shows the thermal conductivity of the PEDOT:PAAMPSA:PA (6.2 wt. % PEDOT content) at different RH values. The thermal conductivity slightly decreased after self-healing.

Measurement of the stress-strain characteristics:
The stress-strain characteristics of PAAMPSA:PA and PEDOT:PAAMPSA:PA free-standing films were investigated through tensile stress tests using a universal testing machine (Tinius Olsen Model H5K-T) at RT and RH of 60%. The free-standing films were prepared using the solvent casting method. The polymer solution was poured into a rectangular silicon mold to obtain 35 × 5 mm free-standing film with a thickness of ~ 0.8 mm. The strain rate was set to 5 mm min -1, and stretching was continued until the free-standing films ruptured. The free-standing films were held by a pneumatic grip. The gauge length of the films was set at 10 -11 mm.
Determination of the power output and energy density: The power output ( ) and energy density ( ) of the PEDOT:PAAMPSA:PA (6.2wt. % PEDOT) ITEC device and ITEC module on stage II were calculated based on the following equations: where is the area of the electrode, is the voltage in stage II, is the load resistance, and ∆ is the duration of stage II.

Supplementary Note 1. Reformulation of Han's theory
The ionic Seebeck coefficient ( ) of iTE materials with two types of carriers is determined by the following equation: where is the electric charge of the ion, is the bulk ion-carrier concentration, is the diffusion coefficient, and * is the Eastman entropy of each ion. 5 Note that of a monovalent ion system is determined by the following equation: where is elementary charge ( = 1.602 × 10 C), and is the Boltzmann constant. is therefore given by Because is reciprocal to the ionic resistance ( ), we can rearrange the equation as follows: Since (note that indicates a thermovoltage at a certain temperature difference) of an iTE material with one type of ion ( ) can be expressed as = * • , then, using Ohm's law where = * is the ionic thermocurrent per unit temperature difference for each ion. For a monovalent system, = − . Thus, can be abbreviated as Here, we define Σ as the net ionic thermocurrent ( , ) and as the net ionic resistance ( , ). In the case of a system containing monovalent cation and anion, the equation is expressed as Finally, we can posit that , is destructive, and , is constructive. S10

Supplementary Note 2. Definition of the structural entropy
The structural entropy (Δ ), which accounts for the changes in the structure of water beyond the hydration shell, is determined by the following equation, where Δ is the entropy gained by the hydration from vacuum to infinitely diluted water, ° is the translational entropy loss, is the rotational entropy loss, and η (0 ≤ ≤ 1) is